Access control refers to the physical security enforced in an area to ensure only the authorized people could access that area. It is very important in a lot of settings, e.g., commercial offices, manufacturing facilities, construction sites, bus stops, train stations, airports, academic buildings, hospitals, stadiums, theaters, and convention centers.
There are several techniques for enforcing access control. In some places, access control is enforced by keeping personnel, e.g., a guard or a bouncer. In some places, mechanical locks and keys are used (only people who have the keys can open the lock). In other places, electronic solutions are used, e.g., using a reader and a badge (swiping the badge into the reader opens the door, if the badge has adequate privilege), passcode based controller (entering the correct passcode into the controller opens the door).